Not sure if you mean how to make camo pattern or how to make plane entirely, so here are both answers:

For base, I would mix three different colors of gumpaste with 2 parts of the predominant grey-blue and one part light blue and another the brighter blue color. Loosely break apart these colors and mush them all into a big ball, try not to mix at all, just press together until they stick. Sometimes wrapping in plastic and letting it rest overnight helps them stick together. Use this as your base.

Roll one larger tube for the body, then roll the ends to points, and cut out a section for the cockpit that gets filled with domed white fondant (paint with pearl powder) to represent the glass (making it with cockpit open is too complex to explain here). Make two smaller cylinders to flank it where the wings attach. Roll flat sheet and cut out wings and tail, push toothpicks into the tail and let dry, then attach with tylose powder. I would not make wheels or missile so it can lay flat on your cake. Once dry paint details with food color mixed with vodka.

@cabralej 's suggestions are great! I made a F-15 Strike Eagle topper fairly recently. I used one piece for the body, made the wings and attachments separately then attached them with tylose glue. I used a 50/50 gum paste/MMF mix for the topper. Mine wasn't camouflage but if it had been I probably would have painted on the camo part. Made the body the most predominant color then paint the rest. Paint the camo, let it dry, then paint any identifiers or squadron specific details after the first layer of camo has dried. It won't let me post a picture but I have one in my gallery of the Strike Eagle birthday cake with the topper. Good Luck!

Hi Cabralej, thanks a lot for explaining detailed step-by-step instructions! I am making the whole body of the plane in camouflage. Yes I will not make those missiles and wheels since the fondant I will use for covering the cake is in light blue colour to represent the sky. I am also planning to make the cockpit closed, I will try the pearl dust, that sounds so fun :D

Wow OHaresTstyTrts, that was a cool looking cake! Thank you for your suggestions, I'll try painting it if all else fails :D